Typically, absorbent sanitary articles comprise an impermeable layer, for example of polyethylene, a layer of non-woven fabric permeable to liquids and an absorbent pad sandwiched therebetween.
These are the basic components of an absorbent article.
In addition to these basic components, absorbent articles comprise further accessory components, such as, for example, elastic bands, faecal barriers and side tabs, which vary the complexity of the structure and production process of the absorbent articles to different degrees.
Prior art machines for making absorbent sanitary articles comprise a line for feeding a continuous web of impermeable material along which there are a plurality of operating units which provide the continuous web with the basic and accessory components needed to make up the finished absorbent article.
Along the feed line, at the operating units, there are a plurality of inspection stations designed to check that the basic and accessory components meet respective quality parameters.
Special attention is reserved for inspection of the absorbent pad of the absorbent article.
Generally speaking, the absorbent pad is made mainly from natural fibres (known as “fluff”) uniformly blended with superabsorbent polymer material (“SAP”)
The absorbent pad may also comprise one or more discrete absorbent layers made mainly or exclusively of superabsorbent polymer material (“SAP”).
In order to check the quality of the absorbent pad of each absorbent article, the prior art teaches the use of an inductive sensor capable of detecting the weight and density profile of the entire pad.
Owing to the high speeds of the feed line, however, sensors of this kind tend to be unstable during machine operation and need to be calibrated frequently.
Moreover, inductive sensors do not allow the weight and density profile of individual absorbent materials making up the pad, such as fluff and “SAP” to be detected distinctly from each other, because sensors of this kind are unable to distinguish each single absorbent material used to make the pad.
With the development of absorbent pads containing less and less fluff and more and more “SAP” the margin of error relating to the contribution, in weight, of the fluff and of the “SAP” in the pad, is more and more reduced.
For this reason, it is very important to be able to check the weight and density profile (or weight profile) of each single material making up the absorbent pad.